ADHD is an early-emerging,1 chronic, 2 and highly impairing neurodevelopmental disorder;3-5 it is also one of the most prevalent disorders of childhood.6 Theories of the etiological determinants of ADHD have focused to a large extent on executive dysfunction in affected individuals.7,8 In turn, interventions for ADHD have been designed to target these putative executive function deficits.9 Unfortunately, this approach has not yielded the results that had been expected.10-12 This may be, in part, because evidence suggests that the executive function deficits observed in children with ADHD may be accounted for by weaknesses in non-executive processes.13,14 Recently, researchers have proposed that when compared to typically developing peers, children with ADHD show less efficient information processing, as indexed by the diffusion modeling parameter drift rate.13,15-19 Furthermore, once drift rate has been taken into account, the magnitude of executive function deficits seen in children with ADHD declines.13,16 Although these findings are promising, we do not know why children with ADHD show less efficient information processing (i.e., lower drift rate). We will recruit 8- to 12-year-olds with ADHD and their typically developing peers into a longitudinal study. Children will be assessed with a neuropsychological test battery at two time points - baseline (T1) and again 9 months later (T2). The first objective of this study is to gain insight into why children with ADHD show poorer information processing efficiency than their typically developing peers. Specifically, we aim to investigate if it is because children with ADHD allocate insufficient effort to meet task demands. To do this, children will perform a choice reaction time task where reward and event rate are manipulated. We hypothesize that compared to TD peers, children with ADHD will show: (i) differentially faster drift rates when trials are rewarded than when they are unrewarded; and (ii) differentially slower drift rates with longer inter-stimulus intervals. Second, to better understand the temporal relation between drift rate and executive function performance, we will examine whether drift rate at T1 mediates the relation between ADHD severity at T1 and executive function performance at T2. It is hypothesized that after including drift rate at T1 in the model, the negative relation between T1 ADHD severity and T2 executive function performance will significantly decrease in magnitude. The present study is innovative in that it is the first to use an experimental paradigm to understand why drift rate is lower in children with ADHD than their typically-developing peers. Further, the longitudinal design helps to clarify the temporal relation between information processing efficiency and executive functioning. Findings from this study may yield important insights for treatment development in the field of ADHD.